1. Field of the Invention:
This invention relates to an anticorrosive coating composition. More particularly, it relates to an anticorrosive coating composition to be used for the production of coating materials, which allows ideal control of the electrodeposition process, and to an anticorrosive coating composition for corrosion-proofed steel sheets which excel in their ability to withstand the electrodeposition process, stamping and spot welding, which exhibits a high level of corrosion resistance and retains excellent anticorrosion ability and adhesion after water soaking of the electrodeposition film, and which can be applied to various kinds of metal (including variegated substances).
2. Description of the Prior Art:
Rustproofed steel plates are copiously used in automobiles and electrical appliances, particularly in doors hoods and other parts of automobiles. The demand for rustproof steel sheets has been growing at an accelerated rate.
The rustproofed steel plate is the result of giving a cold rolled steel plate 1 a zinc-nickel plate 2, subjecting the plated steel sheet to a chromating treatment applied with a roll coater 3, and deposition an organic coat 4 on the chromated substrate as illustrated in FIG. 1. Generally, this rustproofed steel sheet is coated with an electrodeposited film 5 and, if need be, further subjected to an intermediate coat and an overcoat.
The rustproof steel sheet of this construction desirably has an improved ability to withstand the electrodeposition process, stamping and spot welding. Such sheets desirably exhibit high corrosion resistance, and adhesion after the water soaking of the electrodeposition film.
For the purpose of meeting this desire, improvements have been attained concerning coating compositions to be used for rustproofed steel sheets. JP-A-62-73,938(1987), JP-A-62-283,161(1987), and JP-A-64-65,179(1989), for example, disclose methods which rely on incorporation of silica to the anticorrosion composition for the impartation of affinity during electrodeposition. Actually, for the acquisition of the appropriate affinity for electrodeposition, these methods require their anticorrosive compositions to have silica contents of not less than 27.5%. The incorporation of silica in these high concentrations sacrifices the ability of the anticorrosive composition to withstand the impacts of stamping and spot welding and gives rise to the possiblity that the treatment of the anticorrosive composition will be non-uniform owing to the dissolution of silica and separation of the silica from its coating during an alkaline cleaning process. JP-A-62-283,161(1987) discloses a method which relies on incorporation of a hydrophilic resin (such as, for example, polyamide resin) for the impartation of an appropriate affinity for electrodeposition to an anticorrosive coating composition. For the acquisition of an appropriate affinity for electrodeposition, this method requires the anticorrosive coating composition thereof to have a hydrophilic resin content of 5 to 40%. The incorporation of the hydrophilic resin results in sacrificing anticorrosiveness and imposing a limit on the use of a solvent (dibenzyl alcohol) and accordingly jeopardizing the economy of the anticorrosive coating composition. JP-A-50-146,340(1975) and JP-B-58-19,706(1983) disclose methods which involve incorporation of electroconductive pigments and metal pigments. Since these methods require their anticorrosive compositions to contain a metallic powder, zinc oxide, or a metal carbide which inherently has a relatively large particle diameter, the electrodeposition film coats formed of these anticorrosive coating compositions on steel plates suffer from the disadvantage that the surface of the treated plates lacks smoothness because of the unevenness of the coarse underlying particles and lead to problems in stamping. JP-A-62-283,161 (1987), with the same object as this invention, discloses a composition which comprises a urethanized polyester resin, a hydrophilic polyamide, and fumed silica. It further discloses a technique for producing an opalescent metallicized film by the incorporation of melamine cyanurate in a clear coating composition. This composition does not produce an effect suitable for the kind of electrodeposition which is the object of this invention.
The existing anticorrosive compositions, in spite of the latest improvements described above, are far short of producing coating properties on ideally balanced levels. When appropriate electrodeposition properties are obtained in the absolute absence of silica, hydrophilic resin, and electroconductive pigment or in the presence of these components only in extremely small amounts, various means such as variation of the base resin, extensive selection of a curing agent and an anticorrosive agent, and addition of pertinent components may easily be adopted.
An object of this invention, therefore, is to provide an anticorrosive coating composition which obviates the necessity for incorporating silica, a hydrophilic resin, and an electroconductive pigment and allows control of the density of electrification points in a film of a coating material using the composition.
Another object of this invention is to provide a coating composition for use on a rustproofed steel plate, which permits appropriate electrodeposition and excels in stamping, spotwelding, and high anticorrosive ability, and the ability to retain its anticorrosiveness and adhesion after water soaking of the electrodeposition film. Specifically an anticorrosive coating composition which is effectively applicable to various materials inclusive of a plurality of substrates.